you are correct, it is speakers A OR B, never A+B, so never a combined load.
the rear label says 8 ohms (no mention of 4 or 6, just 8).
Remember, a 'nominal' 4 ohm speaker may dip to 2 ohms, that would be even more 'beyond the limits'. find out what you can about the full impedance curve of those 4 ohm speakers you have.
you can try a pair of 4 ohm speakers, try at 50% full volume, no more.
enough sound? ok, a 'safe risk' (a lovely contradiction).
not enough sound at half volume, a bad risk.
taking a 'safe risk'. how do you keep someone else pushing the volume knob too far, which could push the amp too far.
or having the volume up high while using A speaker's which are 8 ohm, perfectly normal: then you or someone else, without noticing the volume setting, switching from A to B, bang, boo boo boo hoo.
IOW, B set 4 ohms speakers might work, but would always be a risk.